1. Field of the Invention
The invention relates to xenograft heart valves and more particularly relates to a porcine mitral heart valve for use in humans and the method for making and implanting the porcine mitral heart valve.
2. Description of Related Art
Heart valves from human cadavers, so called "homograft" valves, have been implanted in living human recipients for at least thirty years. Evolution of the Homograft Valve, Donald N. Ross, Special Presentation, Annals of Thoracic Surgery 59:565-7 (1995). A primary problem with homograft valves is their availability. By contrast, heart valves from animals, so called "xenograft" valves, particularly from pigs, cows and sheep, are abundant. Porcine xenograft heart valves have been collected, treated and implanted in humans to replace damaged or defective human heart valves since at least the early 1970s.
Porcine aortic heart valves have been found to be particularly good replacement heart valves for humans. One reason for this is that porcine heart valves have many similarities in both size and structure to the corresponding heart valves in human hearts.
With respect to the mitral valve, a porcine mitral valve of a certain size generally has cordae tendineae slightly shorter than the cordae tendineae of the human mitral valve it replaces. If a porcine mitral valve of an annular size similar to the human mitral valve it replaces were implanted in a human heart to replace a damaged or diseased human mitral valve, the porcine cordae tendineae would probably not be long enough to reach the papillary heads in the left ventricle. If the porcine cordae tendineae were forced to reach the human heart's papillary heads, undesirable stress would be placed on the subvalvular apparatus. This stress could cause the valve to deform and not perform properly. Alternately, the unnatural stress could cause the mitral valve to prematurely fail. Either of these results is therapeutically undesirable. Therefore, it is desirable in a xenograft mitral valve to mimic the distance from the endocardial wall through the papillary muscles and the cordae tendineae to the annulus of the original human mitral valve.
Throughout this disclosure, the term "subvalvular" means the part of the heart below the annulus of the mitral valve including the cordae tendineae and papillary heads and the term "annulus" means the part of the heart where the leaflets are inserted into the endocardium or inner wall of the heart. In addition, throughout this disclosure, "length of the cordae tendineae" means the distance from the endocardial wall through the papillary muscles and papillary heads, through the cordae tendineae to the annulus of the mitral valve when the mitral valve is implanted.
Many factors can be used to choose the porcine mitral valve to be implanted in the human. Many candidate porcine valves are rejected because their shape, cosmetic appearance, or chordal distribution and geometry are not desirable. However, for porcine mitral valves with the desired shape, cosmetic appearance, and chordal distribution and geometry, if the porcine mitral valve is chosen to provide the appropriate length cordae tendineae, the size of the mitral valve itself will generally be too large to fit the annulus presented from the excised human mitral valve. These are problems in need of a solution.
A method for connecting the cordae tendineae from an explanted xenograft mitral valve to the endocardial wall of a human heart has been tried by M. P. Vrandecic et al. Heterologous mitral valve transplant: the first 50 patients clinical analysis, M. P. Vrandecic et al., European Journal of Cardio-thoracic Surgery, 9:69-74 (1995). Vrandecic's method allows the surgeon to tailor the chordal length of the xenograft from the human mitral valve annulus to the endocardial wall where the papillary heads attach. However, Vrandecic's approach alters the three dimensional structure of the xenograft papillary head. Therefor, the xenograft papillary head is subjected to unnatural stress which can lead to chordal rupture.
Herbert O. Vetter et at. have covered the severed papillary heads of a mitral valve from a sheep with a patch of expanded polytetrafluoroethylene (ePTFE) preparatory to implanting the valve in another sheep. Mitral Allograft with Chordal Support: Echocardiographic Evaluation in Sheep, Herbert O. Vetter et al., Journal of Heart Valve Diseases, Vol. 4, No. 1, pages 35-39, January 1995. The severed papillary heads are attached to truncated papillary muscles in the host heart by sutures through the ePTFE material. Although the ePTFE material provided reinforcement for attaching the severed papillary heads to the truncated papillary heads, this method also does not allow for the length of the cordae tendineae to be lengthened as is required to implant a porcine valve in a human heart and still preserve the human heart's internal geometry.
Thus, the problem still exists as to how to lengthen the length of the cordae tendineae for a porcine mitral valve and maintain a physiologic stress distribution on the xenograft subvalvular apparatus that is implanted into a human heart.